Taking it to the next level

[sploo]

Hi all,

I've been slowly finding my feet with my CNC router, and recently tried cutting the raptor model posted on the excellent Dinosaur skeleton dfx files thread.

The results were pretty good, but I'd like to get advice from those with a bit more experience.

I struggle to get material to stay completely flat to my table surface. It's always firmly clamped at the ends, but if I'm using a large sheet (and filling it with pieces to cut) it's not possible to get clamps around the whole perimeter, or bolts through the middle. At the moment, I stay with the machine during a job and press the sheet to the table near where it's cutting.

This leads on to the fact that, when cutting small pieces such as the dino files, you need some way of stopping them moving as they're cut out. At the moment, I'm using SheetCam's tabs facility, which works pretty well, but this then leaves quite a bit of work cutting the tabs with a knife and cleaning up the parts.

I was considering changing from a single 1/4" deep pass to one most of the way through, then a final shallow cut - this way I'd reduce the force of the cutter on the parts and may be able to get away with smaller, thinner, tabs.

The problem of course is that this would increase the job time, which is no fun when you're standing over the machine, pressing the material down!

I was wondering if several strips of double sided tape (between the workpiece and the sacrificial board) would work? They'd have to be strong enough to hold the material flat (even better if it's strong enough to hold the cut parts, so I don't need tabs) but would have to be easy to remove afterwards (from wood and MDF). I guess it'd also have to not gum up the cutting bit, and be thin enough to not seriously affect depth accuracy. Not asking for much am I?

On a final note, I've been using both up and down cut spiral bits (1/4" diameter). Whilst they cut well, I do get quite a bit of 'fluffy' edging on parts cut from MDF. This is only on the top/bottom surface and is easily removed with a bit of sandpaper, but it is time consuming when you've got lots of parts. Is this just what happens, or is this indicative of rpm/linear speed too slow or fast?

Any thoughts from 'them wot know' greatly appreciated!

[ger21]

The best way to hold parts flat is with a vacuum hold down table, or vacuum jigs made for individual parts. Even with vacuum, small parts are always a problem. Leaving a very thin layer for the final pass is called onion skinning, and can work well, but again, small parts are always going to be a problem. Double sided tape will also work well, but you may go through a lot of tape and it's a pain to get the tape off.

If you're getting tired of standing over the machine waiting, you need a faster machine.

And lastly, you want to get a compression spiral bit, which will cut cleanly on both the top and bottom edges, but only if you cut full depth in a single pass. Or, make multiple passes and leave a small amount of material for a full depth cleanup pass.

Sorry for the brevity, but I'm a bit busy right now.

[sploo]

The best way to hold parts flat is with a vacuum hold down table, or vacuum jigs made for individual parts.

I was afraid you were going to say that

User 'xlr8tr@mac.c' has a site (http://www.morgancustomguitars.com/index.html) with images of his guitar work - very nice BTW. One of the photos (attached) shows what I thought was a vacuum table, but I'm now wondering if it's a downdraft table.

I could make something like this - a box, maybe 2-3" deep, with slots on the top. Do vacuum tables require large airflow (i.e. dust extractor power) or high pressure (i.e. vacuum power)? I'd suspect the latter.

Even with vacuum, small parts are always a problem. Leaving a very thin layer for the final pass is called onion skinning, and can work well, but again, small parts are always going to be a problem. Double sided tape will also work well, but you may go through a lot of tape and it's a pain to get the tape off.

OK. Sounds like I need to think of the vacuum unit as a way of holding the material flat (so I don't need to be with the machine) and still rely on tabs and hold downs.

If you're getting tired of standing over the machine waiting, you need a faster machine.

*LOL* Some of the figures I've seen you quote (400ipm, single pass on thick material) are crazy.

I don't have chip load figures for the cutters I'm using, but took some averages for MDF. With a 2 flute cutter, and approx 12000rpm, a chip load of 0.008 gives me around 200ipm (1/4" bit).

Ron at K2 told me the machine would do 150ipm with a 1/4" bit, 1/4" deep pass. I tried 120ipm, but my control box keeps cutting out. At 80ipm it worked OK.

Perhaps the 150ipm figure he mentioned was for 'simple' straight line cuts - the movement of the router head at 120ipm was pretty aggressive.

And lastly, you want to get a compression spiral bit, which will cut cleanly on both the top and bottom edges, but only if you cut full depth in a single pass. Or, make multiple passes and leave a small amount of material for a full depth cleanup pass.

Understood. I was doing some shallow (1/16") passes in some MDF with a downcut bit and was quite surprised to see 'fluffy' edges on the top. I wasn't cutting all the way through though, so that might be why.

I've not yet found a decent source for compression bits here in the UK - the one I've found was a 1/2"... for over 100USD

Sorry for the brevity, but I'm a bit busy right now.

For a brief post you answered a lot of questions - thanks!

You wouldn't like to get a plane over to Oxford and perform a complete brain dump of everything you know?

[ger21]

Understood. I was doing some shallow (1/16") passes in some MDF with a downcut bit and was quite surprised to see 'fluffy' edges on the top.

Dull tool.

That looks like a downdraft sanding table, btw.

[sploo]

Dull tool.

That's worrying - it was brand new!

That looks like a downdraft sanding table, btw.

That's what I thought. I expect you'd get some reasonable suction as a hold down from that setup, but probably not enough.

I was thinking about the practicalities of a vacuum table, and I think I might try a different approach. My table top has a tapped hole every 4" or so (using tee nuts) - so if I created a bit of gcode to cut a grid of holes in a sheet before running my intended job I should be able to bolt it down pretty well.

I'll have to create a 'mask' with the holes so I can lay out parts around the bolt heads, but I guess it's better than standing over every job. Still not ideal for larger things, but for a sheet of small parts it might work.

It's also just occurred to me that I could use a bearing guided bit on the router table to remove the tabs on the parts - as long as there's enough depth of material for the bearing to follow I guess.

[ger21]

OK, you have a new bit. If you make a single cut through some mdf, do you see that 1 side of the cut is cleaner than the other? The left side should be a nice sharp edge, and the right side of the cut might be a bit fuzzy. Because of this, it's usually desireable when cutting wood to always cut out parts in a counter clockwise direction (conventional cutting) vs clockwise (climb cutting). The exception would be that when cutting hardwoods, you sometimes need to climb cut to prevent tearout. And remember that when doing pockets or cutouts, you travel in the opposite direction. CCW for outside cuts, CW for inside cuts.

[CrashMasterKev]

Hey sploo, I was just reading your post and thought I would return the favor as you have helped me in the past, “Problems with new machine” post, I haven’t been on here in a while as I finally got things up , running and heading in the right direction so I’ve been busy doing my thing in the garage, it’s been fun and is going really well, the best thing I’ve done is add a vacuum pump to my system, Well, I changed out my aluminum table to mdf and leveled/ flycut/ surfaced, whatever you want to call it it, then I set up the vacuum system, it really works great.

I’ve been operating and programming for about 10 years, and having tried many ways to hold parts down, Vacuum is the way to go for me, tape stinks but has its place i guess, I’ve used dust collectors, but that’s all that they’re good for at that point and i'm sure you know the issues with a wet and dry vac.
This company has a lot of good products, we use them at work regularly and I’m of coarse using it at home to.
http://www.allstaradhesives.com/
Just my 2cents, Crash

[sploo]

OK, you have a new bit. If you make a single cut through some mdf, do you see that 1 side of the cut is cleaner than the other?

Yea - when I use an upcut bit I do get a pretty clean bottom edge, but the top is obviously a bit fuzzy.

When I use the downcut bit I was surprised to still get a bit of mess at the top - but I will admit I don't have any decent dust extraction yet, so the amount of wood chips around might have caused a problem. I'm actually making ducting parts with the machine right now!

I guess I need to get hold of a 1/4" compression bit and try that. Not found a UK source yet unfortunately.

The left side should be a nice sharp edge, and the right side of the cut might be a bit fuzzy. Because of this, it's usually desireable when cutting wood to always cut out parts in a counter clockwise direction (conventional cutting) vs clockwise (climb cutting). The exception would be that when cutting hardwoods, you sometimes need to climb cut to prevent tearout. And remember that when doing pockets or cutouts, you travel in the opposite direction. CCW for outside cuts, CW for inside cuts.

I've been using routers (hand held and on a table) for quite a few years, so I'm well practiced in the 'directions of cutting'... including what happens if you push something from the wrong side on a router table... wood gun anyone?

SheetCam does a good job of ensuring the cut direction is correct, though I have to flip it on occasion if I'm doing something special. I didn't know it was called climb cutting though, so it explains that tick box (thanks).

Interesting info about using it for hardwoods. Not something I've done yet, but I do plan to try some hardwood cuts at some point.

Since my last post, I've tried using a bearing guided bit on the router table to clean tabs off some parts - it works very well!

Merry Xmas all!

[todd71]

I use double sided tape with a few screws here and there. I still leave a couple of tabs/bridges. One on each side of where the tool path start point is. It's worked pretty good. I cut the raptor puzzle out of 3/4 inch sheet of MDF with a 1/2 compression bit in 1 pass using this technique. http://www.brontapes.com sell's sheets of double sided film in sizes up to 4ft wide. Works great.
Just cut the tabs with a chisel and peel off the film.To remove the rest of the tabs off the pieces. Set up a "table router" using a piloted straight sided bit to ride around the profile.
Good luck
Todd
http://www.innovative-accents.com

[sploo]

I cut the raptor puzzle out of 3/4 inch sheet of MDF with a 1/2 compression bit in 1 pass using this technique.

Excellent! How much spindle HP does that need?

I have found a place where I can get a similar bit here in the UK. Price in USD as of today (wait for it)... $214!

I scaled him down to fit on my table on one go, and cut from 1/4" material. I see that the result in 3/4" is a monster! (bad pun intended)

A problem I see with cutting this type of part is how you'd do it economically. I'm not in business, but it occurs to me that you couldn't charge a huge amount of cash for one of these dinos, but there's quite a bit of 'post cutting' work cleaning up the parts.

[ger21]

I think you misunderstood what I was trying to say. When I said 1 side was cleaner than the other, I didn't mean top and bottom. What I was trying to say, was, when using a downcut spiral, you can still get fuzzing on the top edge if your climb cutting. Conventional cutting will almost always give you a sharper, cleaner edge.

[sploo]

Ah yes, OK. Both for CNC and manual I always ensure I conventional cut, to ensure the face on the part I want is clean. Or at least, should be...